How can I shatter a model and have it reassemble in a different shape?
One tactic that might be useful is to have the two models both be shattered, but the second one in reverse.  In the time between slowly replace the pieces of the first model with the pieces of the second (which will be hard to notice in all the confusion).  So the first model shatters; over time its pieces are replaced by pieces of a second model; which then unshatters.